1. Field of the Invention
Systems and methods consistent with the present invention relate to a time synchronization method in the wireless sensor network. More particularly, the present invention relates to a time synchronization method in the wireless sensor network, in which an upper node provides back-off scheduling to lower nodes in the wireless sensor network of a hierarchical structure, and each of the lower nodes synchronizes time according to the back-off scheduling based on its local clock.
2. Description of the Related Art
In general, an ad-hoc network is a representative example of a wireless sensor network. The ad-hoc network does not have a central managing system. Instead, the ad-hoc network uses an existing communication-based structure and has a fixed controller for connection between mobile nodes such as a router, a host, and a wireless base station. That is, in the ad-hoc network, a mobile node functions as a router. Therefore, when a mobile node wants to communicate with a counterpart, the mobile node has to develop a communication path via a plurality of nodes between the mobile node and the counterpart.
For example, in an ad-hoc network, there is a wireless network of a tree routing structure. The tree routing structure is widely used due to the strong points of address allocation and routing among functions of the network. That is, in the network of the tree routing structure, addresses are sequentially allocated according to the tree layer, and a packet is transmitted along the tree path.
The ad-hoc network is synchronized by an upper node which functions as a mediator. For example, in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, all of the lower nodes connected to the ad-hoc network can be a mediator which synchronizes the ad-hoc network every beacon frame. In addition, in the 802.15.3 standards, one mediator exists while the ad-hoc network is maintained, and beacons frame periodically generated by the mediator are broadcasted to each of the lower nodes and synchronized.
Thus, an upper node broadcasts data for synchronization to each of the lower nodes, and transmits and receives the data for synchronization to and from each of the lower nodes as many as the number of the lower nodes. For example, if an upper node is connected to lower nodes of a number of n, the upper node uses packets of 2n for synchronization. Accordingly, power consumption increases according to data transmission and reception for synchronization.
Furthermore, as there is no mechanism for managing a synchronization level of the network, only a synchronization technology regardless of synchronization of a desired level exists. Therefore, unnecessarily precise synchronization regardless of a required level of a synchronization error which varies according to an application, and unnecessarily repeated synchronization causes overhead or power loss.